With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 includes, in axial flow series, an air intake 1, a propulsive fan 2, an intermediate pressure compressor 3, a high pressure compressor 4, combustion equipment 5, a high pressure turbine 6, an intermediate pressure turbine 7, a low pressure turbine 8 and an exhaust nozzle 9.
Air entering the air intake 1 is accelerated by the fan 2 to produce two air flows, a first air flow into the intermediate pressure compressor 3 and a second air flow that passes over the outer surface of the engine casing 12 and which provides propulsive thrust. The intermediate pressure compressor 3 compresses the airflow directed into it before delivering the air to the high-pressure compressor 4 where further compression takes place.
Compressed air exhausted from the high-pressure compressor 4 is directed into the combustion equipment 5, where it is mixed with fuel and the mixture combusted. The resultant hot combustion products expand through and thereby drive the high 6, intermediate 7 and low-pressure 8 turbines before being exhausted through the nozzle 9 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors and the fan by suitable interconnecting shafts.
Modern fuel injectors 18 now commonly include an injector head 22 mounted on a stalk 20. The head 22 has a set of pilot nozzles adapted to eject fuel at a low power requirement and a set of main nozzles adapted to eject fuel into the combustor at cruise and at higher power levels. Typically the pilot nozzles also supply fuel to the combustor at high power requirements. The use of two sets of nozzles provides a highly efficient injector with acceptable emissions.
Fuel is supplied to both sets of nozzles through tubes running the length of the injector stalk. A check valve is located at the end of the stalk opposing the head end. The check valves hold fuel in their upstream manifolds in order to avoid having to prime the manifolds following a demand from the engine controller for more power.
One of the problems with having stagnant fuel in the injector fuel galleries is that at operational temperatures of the engine the fuel can undergo thermal breakdown leading to deposition of carbon in the manifolds in a process known as coking. Heavy coking can block the fuel passages causing, ultimately, failure of the injector amongst other problems.
The fuel upstream of the valve is located in a position cool enough not to suffer from coking and the flow of the pilot fuel is sufficient to keep the temperature of the stagnant fuel below the coking temperature. Downstream of the valve the temperature is high enough for the fuel to coke and it is desirable to purge the galleries and conduits downstream of the valve to remove stagnant fuel.
In U.S. Pat. No. 5,243,816 a valve is provided and is opened and closed by the pressure difference between the fuel supply and the pressure of air coming from the compressor and upstream of the injector. A spring is used to bias the valve into the fuel off position. As will be appreciated from this document whenever the fuel is not flowing into the injector head the purge air is continually flowing through the injector. This arrangement is acceptable in an injector having just one set of nozzles e.g. pilot or main, but in an injector having both sets the continual flow of hotter air through the galleries will increase the temperature of fuel flowing (sometimes slowly) in adjacent galleries above its coking temperature and heatshields which are provided to avoid this situation are bypassed.